Wearable communication technology has resulted in a growing demand for compact and miniature antennas. Some wireless smart communication devices (e.g., wearables, smartwatches, etc.) may be worn on the wrist or near a human body. The term “smart” arises because some of these devices (i.e., smartwatch) can be connected to the Internet. They can synchronize information with smart phones or tablet PCs. Conventional smartwatches operate generally in the frequency bands of Bluetooth® (BT), Wi-Fi®, and Global Positioning System (GPS).
To become independent of smart phones, the next generation of smartwatches or advanced wearable wireless devices, require operations over additional frequency bands such as Global System for Mobile Communication (GSM), Third Generation (3G), Fourth Generation (4G), and the future Fifth Generation (5G). This sets new challenges for small antenna designs and the demand of the increased in bandwidth (BW) requirements.
Fourth Generation (4G) was developed to meet the demand for higher data and was introduced using the LTE-A (Long-Term Evolution Advanced) standard which targets to support as high as 100 Mbps data rate for high mobility and up to 1 Gbps data rate for end-users. The specifications require the mobile terminal antenna to operate within the 700-960 MHz and 1700-2700 MHz frequency bands. The biggest challenge is to cover a relatively large bandwidth of 30% in the 700-960 MHz (i.e., Low-Band (LB)) since the physical space reserved for the antenna in a typical mobile terminal is electrically small compared to the free space wavelength at 700 MHz. Mobile antennas are now required to provide multi-band operation while dealing with the constraints including physical limitations and limited volumetric space for integration with other device components. A low profile and low weight design is preferable to ensure slim multifunctional platforms. The most common antenna designs for mobile devices are internal antennas such as patch/PIFAs (planar inverted F Antenna) and monopoles. Nevertheless, they are constrained by the fundamental limits of small antennas that imply an inherently narrow BW. There is a minimum achievable Q (quality factor), and thus a maximum BW for a given volume assigned to the antenna. For PIFAs, several well-known techniques are used to provide dual-band or multi-band operation such as inserting slits in the radiating path or using slotted ground planes. These approaches increase the complexity of the design and make it difficult for integration in slim platforms. The antenna must also be arranged at a certain height with respect to the ground plane occupying a considerable volume to guarantee good performance.
The focus of antenna designs has been on their geometry. The antenna is typically a self-resonant element that provides an efficient radiation pattern independent from the ground plane structure. However, the relevance of the ground plane in the radiation process has been underestimated and is progressively acquiring relevance since studies have demonstrated its strong contribution to the radiation properties of antennas. Several approaches have been proposed to incorporate the use of the ground plan for increasing BW for multi-band operation. These include the use of Coupling Elements (CEs), with various configurations, to capacitively excite the currents on the ground plane of the Printed Circuit Board (PCB), exploiting the low-quality factor of the wide ground plane, to achieve the LB requirements. The antenna impedance is then tuned to cover the desired frequency bands (e.g., LB and or HB) with the use of one or more Matching Network (MN). However, dual-band operation is challenging to obtain, requiring complicated design of the MNs, using large number of device components (e.g., surface mount devices) in a limited amount of space, increasing losses due to the internal resistances, increasing cost of the antenna, and higher power requirements. The size of the batteries is constrained by the total size of the device and becomes an issue for miniaturization.
Another important aspect of the design of antennas is the surrounding operating environment of the antenna. The effect of the proximity of the antenna to the human appendages (e.g., arm, wrist) is therefore an important parameter. Antennas may have performance degradation when in contact with or in proximity to the body which is a lossy dielectric medium. The small size of the antenna in proximity to the human body greatly reduces the radiation range. Some antenna designs, incorporated within the body-worn device or within the wrist band, may result in an undesirable amount of body absorption, high specific absorption rate (SAR), shift in resonant frequency (i.e., detuning), altered radiation pattern, decrease in radiation efficiency, and return loss of electromagnetic signals.
Therefore, the expectations for advanced wearable wireless communication devices present considerable product development challenges in the selection, design, and incorporation of antennas, including power management. The antennas must be designed with specific requirements to ensure suitability, proper function, and simultaneously meet several performance criteria. They must be compact, low-profile, omnidirectional, in some cases highly directive, low specific absorption rate (SAR), easy to manufacture, low lost, and multiband operational. Lightweight designs for improved comfort are also demanded for those designs to be worn particularly around the wrist, which is the most convenient location. Moreover, computing units, sensors, battery cells and related electronic parts are integrated in those devices. These impose a challenge due to a very limited space allowed in the wrist-worn device for the embedded antenna. However, such a design may be constrained by, for example, a small ground plane, limited volumetric space, thus limiting the ability of wearable to meet functional and operational requirements, especially for multi-band operation.